This invention relates to telephone line cards and, in particular, to a control loop circuit utilizing field-effect transistors which allows the precision of the resistance of the field-effect transistor to be determined by external resistors.
Telephone line cads are typically coupled between an external phone line and the control switches in a central office. The telephone line card typically provides two terminals, the TIP and RING terminals, to an external phone, whereby a first signal is applied to the TIP terminal, while a second signal is applied to the RING terminal. Further, a first resistor and a first mechanical relay are typically coupled between the TIP terminal and the central office, while a second resistor and a second mechanical relay are typically coupled between the RING terminal and the central office. The mechanical relays are typically closed under normal operation and normal voice signals typically appear as differential signals across the TIP and RING terminals. It should be pointed out that the first and second resistors must be closely matched, typically on the order of 0.1% of each other, so that when a common mode signal is induced onto the TIP and RING terminals, there is substantially no differential voltage between the TIP and RING terminals which would interfere with normal voice signals. In other words, the matching of the first and second resistors should provide substantial common mode rejection.
The problem of having the first and second resistors closely matched is further complicated when protection against lighting surges is desired. For example, when lightning strikes, the voltage on the TIP or RING terminals can quickly rise to an excess of 1,000 volts. Typically, the first and second mechanical relays are too slow to respond to this lightning surge and, therefore, the first and second resistors must dissipate the large power generated from the lightning surge. This means that the first and second resistors must be high power resistors as well as closely matched to each other. Furthermore, high power resistors are expensive and difficult to fabricate on an integrated circuit.
One solution to this problem of closely matching high power resistors is to replace the first and second mechanical relays with first and second field-effect transistors so that when lightning strikes, the FETs will turn off and the first and second resistors will not have to absorb the large power generated by the lightning surge. However, this approach requires the use of precision FETs that closely match each other in order to maintain substantially zero differential voltage between the TIP and RING terminals as aforedescribed.
Hence, a need exists for a circuit that controls the resistance of a field-effect transistor.